A method of producing methacrylic acid by subjecting methacrolein to gas phase contact oxidation on a catalyst comprising a compound oxide containing molybdenum and phosphorous as essential components is widely known and is also used on an industrial scale. In this case, methacrylic acid is often produced at a reaction temperature of 250 to 400° C., using of a catalyst as a fixed bed.
The catalyst for use in such a gas phase contact oxidation reaction is used for a relatively long time, but usually the catalyst deteriorates with time. Causes of the degradation of the catalyst include reduction of a catalyst component, sublimation and flying of a catalyst component, change in a crystalline phase in a catalyst structure, and the like.
As for methods for regenerating the thus deteriorated catalyst, a variety of propositions have been made. For example, Japanese Patent Laid-open No. 58-156351 discloses a method in which the deteriorated catalyst is treated at a temperature of 70 to 240° C. in a gaseous stream having a water vapor partial pressure of 10 vol % or more, and Japanese Patent Laid-open No. 6-7685 discloses a method in which the deteriorated catalyst is thermally treated under a stream of an oxidizing gas containing 0.1 vol % or more of molecular oxygen at a temperature of 300 to 410° C. for 0.5 to 50 hours. From an industrial viewpoint, however, it can be considered that it is desired to minimize the frequency of regenerating the deteriorated catalyst, because it is quite inconvenient to regenerate the catalyst each time it deteriorates and the production of methacrylic acid must be temporarily suspended every such an occasion.
That is, from an industrial viewpoint, there is desired the method of producing methacrylic acid while the deterioration of the catalyst is inhibited as much as possible, rather than the method of regenerating the deteriorated catalyst.
Furthermore, as for a catalyst that is resistant to the deterioration and has a long catalyst life, and a method for producing the same, numerous propositions have been made, for example, as in Japanese Patent Laid-open No. 5-31368. However, a level of the inhibition of the catalyst deterioration is not necessarily sufficient in industrial practice, and further technological innovation is desired. Moreover, all of these propositions only demonstrate that a deterioration rate is slightly reduced as compared with the catalyst by the conventional production methods under the same reaction conditions, and they do not refer to a method for effectively inhibiting the deterioration of the catalyst by controlling reaction conditions within specific ranges.
In addition, as the reaction conditions in producing methacrylic acid by passing a material gas containing methacrolein, oxygen and water vapor through a reactor packed with a compound oxide catalyst containing molybdenum and phosphorus, the following are known.
That is, Japanese Patent Laid-open No. 9-75740 describes that a concentration of methacrolein in a reaction material gas is in a range of 1 to 10% and a ratio of oxygen to methacrolein is about 1 to 5, and a space velocity of the material gas is preferably in a range of 500 to 5,000 h−1, and water vapor or the like can be used as a diluting gas. In examples described in this publication, a material gas having a composition of 4 mol % of methacrolein, 12 mol % of oxygen, 17 mol % of water vapor and the balance of nitrogen is passed through a reactor at a space velocity (STP standard) of 670 h−1 so that methacrolein may be subjected to the contact oxidation reaction.
Furthermore, Japanese Patent Laid-open No. 9-313943 describes that an appropriate contact time of the material gas is 1 to 20 seconds, and as for the composition of the material gas, 0.2 to 4 mols of molecular oxygen and 1 to 20 mols of water vapor are used with respect to 1 mol of methacrolein. In examples described in this publication, a material gas having a composition of 5 mol % of methacrolein, 12 mol % of oxygen, 30 mol % of water vapor and 53 mol % of nitrogen is passed at a space velocity of 1,400 h−1, or a material gas having a composition of 3.2 mol % of methacrolein, 16.4 mol % of oxygen, 19.4 mol % of water vapor and 61 mol % of nitrogen is passed at a space velocity of 2,060 h−1 so that methacrolein may be subjected to the contact oxidation reaction.
However, the reaction conditions described in Japanese Patent Laid-open No. 9-75740 and Japanese Patent Laid-open No. 9-313943 are reaction conditions in a case where a long-life catalyst is used, and the publications only disclose very wide ranges of the material gas compositions and the space velocities. These publications do not suggest techniques for inhibiting the deterioration of the catalyst by adjustment of the material gas composition and the space velocity anywhere. Moreover, in a case where methacrolein is subjected to the contact oxidation reaction using the material gas composition and the space velocity specifically described in these publications, the deterioration of the catalyst cannot be effectively inhibited.